The field of the present invention covers the fertilization that takes place through the soil, the latter being meant as the natural means to make the fertilizer available to the root apparatus. Thus it entails preventing the fertilizer, constituted by a common salt, for instance ferrous sulfate (FeSO.sub.4) placed in a soil lacking iron, with a neutral-alkaline pH (e.g. pH 7-8) and poor in humic substances, from transforming itself into insoluble products such as oxide, hydroxide and carbonate.
From an ideal point of view, a fertilizer based on meso and micro elements to be used for soil fertilization should have the following characteristics:
Currently there are three main categories of fertilizers based on meso and micro elements:
Said complexes very often do not allow a good utilization of the metal both due to biodegradability through micro-organism attack, and due to the excessively intense bond force, which does not allow the metal to be freed and inserted in the biological cycles;
The possibility is known of using as organic salts some derivatives of the tartronic acids and their salts in the variants with and without phosphate mobilization, as Italian patents no. 767605, no. 788509 and no 833128 teach. Other chelates and organic salts are described in U.S. Pat. Nos. 4.181.516 (Gray, 1980) and U.S. Pat. No. 4.786.387 (Marihart, 1988) and in European patent 284339 B1 (Marihart, 1991). In these last two patents fulvic acid (a fraction of the humic substances) is used together with various chelates and anhydrous ammonia (in order to neutralize the mixture) without obtaining the formation of protective bridge structures of the type that is the subject of the present invention.
A work published by D. F. Clemens and colleagues in Fertilizer Research Vol 25, pp. 127-131 (1990) describes the structure of chelating agents and of the related metal complexes, such as those of Fe(III), distinguishing it in two classes: aminopolycarboxylates and hydroxycarboxylates (where the term carboxylate means "functional group of an organic acid"). For instance to the aminopolycarboxylate class belong EDTA (ethylendiamintetracetic acid), and also EDDHA (ethylendiamine di(o-hydroxyphenylacetic) acid); to the hydroxycarboxylic class belong, for instance citric acid, gluconic acid, glucoheptonic acid.
The structure of the humic substances corresponds to the characteristics required by an ideal chelating-complexing agent. For the purposes of vegetable nutrition, humic substances are in any case the ideal complexing agents for iron and for the other nutritive elements, as is known from the technical literature, for example according to F. J. Stevenson "Humus Chemistry" J. Wiley & Sons Ed. (1982), according to P. Sequi "La sostanza organica nel terreno. Funzioni nutrizionali" L'Italia agricola 113(2):91-112 (1976).
The structure of the humic substances is not well defined in the part the involves the chelation of metals. It is however known that the humic molecule uses mainly the OHs of the functional groups to be able to complex the metal elements (Stevenson, 1982). According to some authors the structure of the metal-humate complex involves two oxyhydrils, one of them carboxylic and one deriving from another functional group, alcoholic, phenolic or other. As reported by Sequi (1976), the structure of the complex also comprises a phosphate group that in this case has two functions:
In this respect it should not be forgotten that phosphate also presents great problems of availability in the soil since it is easily prone to undergo precipitation processes.
According to Stevenson (1982), the extraction of the humic substances using a base such as NaOH or KOH takes place through the salification of the humic and fulvic acids in an alkaline environment according to the following reaction: EQU HA--OH+K--OH.fwdarw.HA--OK+H.sub.2 O
where HA--OH is the humic acid. The K.sup.+ can also be replaced with Na.sup.+ ; the compound that is formed (HA--OK or HA--ONa) is a salt of the humic acid (humate).
In the case wherein to the humic molecule is bonded a non alkaline metal (e.g. iron), the base alone is not able to salify the humic acid and the extraction of the humic acid complexed with the metal has to take place with the aid of a salt that possesses a chelating agent, such as sodium or potassium pyrophosphate. According to what is reported by Stevenson (1982) the reaction that would take the humic acid into solution could be as follows: EQU HA--O--Me--+1/4K.sub.4 P.sub.2 O4.fwdarw.HA--O--K+1/4Me.sub.4 P.sub.2 O.sub.7 .arrow-down dbl.
In other words the metal would be detached from the humic acid and replaced with the K.sup.+, thereby originating a salt of the humic acid, whilst the metal would be complexed by the pyrophosphate originating an insoluble salt which would precipitate (.arrow-down dbl.).
The aforesaid reaction is likely to take place only if the complexing agent of the iron has a bonding force weaker than that of the pyrophosphate, but it cannot take place when the organic complexing agent (humic substance in the specific case) has a bonding force stronger than that of the inorganic complexing agent (pyrophosphate).
In a process of extraction of the humic substances, i.e. of solubilisation, the combined action of the two extracting agents KOH and K.sub.4 P.sub.2 O.sub.7 allows to realize the following phases (Stevenson, 1982; E. Bar-Ness and Y. Chen Plant Soil, 130, 35-43, 1991):
In the occasions mentioned above and in other cases wherein the attempt was made to prepare metal chelates based on humic substances for research or commercial purposes, in order to have available a product with higher quality and performance than those of the products present on the market, the following problems were noted: